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ZnO Quantum Dots Essay Examples & Outline

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ZnO QUANTUM DOTS

Objective

The main objective of this experiment was to synthesize Nano scale particles for Zinc Oxide (ZnO) to study their growth over time and describe its three dimensional quantum confinement.

Introduction

Quantum dots are zero-dimensional nanoparticles of semiconducting material whose excitons are confined in three spatial dimensions. As a result of their small size, they exhibit optical and electronic properties that lie between those of bulk solids and those of discrete molecules. Nanocrystals are capable of absorbing light and remitting it in a different colour. The absorption wavelength by the nanocrystals is usually a function of the size of particles at the Angstrom scale. When an electron in a bulk semiconductor is excited from the valence to the conduction band, an electron “hole” of opposite electric charge is left behind.

Due to the Coulomb force between the excited electron and the “hole”, the excited electron is attracted to the “hole” to form an electron-hole excitons or pair. In a bulk semiconductor, this electron-hole excitons are typically bound within the Bohr characteristic exciton radius. By constraining the electron-hole pair further causes changes in the properties of the semiconductor. This effect is a form of quantum confinement that is a key feature in a range of emerging electronic structures.

The effect specifically describes the phenomenon results electron holes and electrons being constrained to a critical quantum measurement. Under confined conditions, the quantity of energy that can be possessed by the electrons is quantized to discrete amounts, rather than the continuous bands in the bulk solid. When the hole and the electron recombine energy is released in the form of a photon. The photon is emitted with discrete energy values and wavelengths.

In addition, the difference in the electron energy levels depends on the degree of quantum confinement. A greater degree of quantum confinement produces larger gaps between the energy levels, which result to emission of photons with smaller wavelengths. For the case of semi-conduction nanocrystals, quantum confinement can be explained using a 3-dimentional “particle-in-a-box” model (Bhattacharya, Fornari and Kamimura).

Quantum dots are significant for optical applications, particularly because of their high quantum yield. They have proven to function like a single-electron transistor, and have also shown the Coulomb blockade effect, thus, making them to find applications in electronics. In addition to these, quantum dots have been suggested as implementations of qubits for processing of quantum information (Rodriguez and Fernández-García).

In this lab experiment, nanocrystals of ZnO were prepared by reacting zinc acetate dehydrate, Zn(C2H3O2)2.2H2O, with tetramethyl ammonium hydroxide pentahydrate, N(CH4)4OH.5H2O. The growth of ZnO nanoparticles was then monitored by measurement of the UV-vis absorption spectrum at wavelength 230 to 390 nm as a function of time. The first sample was not placed in the ice bath, the second one was not. The absorption spectra was then acquired at intervals of 6 minutes after initiation of the formation of ZnO nanoparticles over a duration of 90 minutes. The solutions were then sealed and held for 2 days, and the final absorption spectra was measured.

Experimental Procedure

Please refer to the lab manual “ZnO Quantum Dots”, page 3.

Analysis of Results

Absorption Spectra

The size of the ZnO nanoparticles is calculated using the empirical formula shown below:

One unit of angstrom is equivalent to 0.1 nm. This means that the size of our ZnO nanoparticles range from 1.4055nm to 1.44860nm. The ideal particle size usually ranges from 2.0 to 7.8 nm. This means that the size particles in this experiment are slightly smaller than the ideal size. The size of particles are controlled by a number of factors, mainly the temperature of synthesis. Temperature control is very significant in controlling the size of particles produced.

The graph in figure 5 below shows how particle size of ZnO varies with time. From this graph, it can be observed that the particles grow slightly larger as time moves. This trend is evident within the first 60 minutes. Beyond this time, the particle size tends to remain constant. However, the variation in particle size is very narrow, with all particles being within a range of 14.055 to 14.5115 angstroms.

From the results in figure 6, it shows that the band-gap energy increases as the particle radius (R-2) increases. In other words, as the particle radius increases, the band-gap energy begins to fall.

As observed in the results obtained for sample 1 (no ice), the results for sample 2 presented in table 2 above show that the particle size grows with time. An interesting observation in this result is that the particles are smaller in size compared to the size of particles in sample 1. This means that temperature plays a significant role in the formation of ZnO nanoparticles.

The Effect of Temperature on the Growth of the Particles.

Synthesis of three dimensional ZnO nanostructures through the hydrothermal process by use of ammonia and zinc acetate at a temperature of 65oC yields successful results. The temperature during the synthesis process has an effect on the particle size, band gap, sensitivity and structural defects. Increase in temperature beyond the required temperature results in increased size of particles. Higher temperatures cause increased rates of growth dynamics, and the later introduce defects in the nanostructure, hence, reducing the crystal quality (Tartakovskii).

Conclusions

The temperature plays a very significant role in the formation and growth of ZnO nanoparticles. At a lower temperature, the particles formed are slightly smaller in size compared to a higher temperature. The particles increase in size as time increases after they are formed. As the particle size increases, band-gap energy increases.

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